Polymer blends represent an important class of polymeric materials, accounting for 20% of the U.S. synthetic resin market in 1985. Although the majority of commercially important polymer blends are thermodynamically immiscible heterogeneous blends, considerable interest exists for producing miscible homogeneous blends, because such blends achieve properties intermediate to those of their constituent polymers. Commercial miscible blends are at present mainly used for their mechanical and thermal properties, but immense potential exists for the development of polymer blends with specially tailored optical, surface, barrier or biodegradation properties. In spite of much synthetic research effort, miscible polymer pairs are rare, because the high molecular weight of polymers provides little entropic driving force for miscibility. Favorable enthalpic contributions, which lead to miscibility, are possible only when specific interactions are present between moieties in the different polymers.
In an attempt to bypass the specific interactions requirement, researchers have studied potential techniques for producing nonequilibrium single-phase blends of thermodynamically immiscible polymers. These techniques typically involve the preparation of a dilute ternary solution of two polymers in a common liquid solvent, followed by rapid quenching of the blend into the solid state via coagulation, solvent evaporation or freeze-drying. These diffusion-governed processes are slow, as compared with incipient polymer phase separation in liquid solutions, so it is not surprising that some degree of phase separation occurs, e.g., micron-sized domains are formed or extremely broad glass transition temperatures are noted.
In light of the fact that many polymers (e.g., all glassy polymers) enjoy commercial use in a thermodynamic state far removed from equilibrium, it is short-sighted to limit the search for intermediate-property polymer blends to systems that are thermodynamically miscible. For example, rapid coagulation or quenching of polymer mixtures can result in a frozen-in single phase morphology, such as that observed in molecular composites. However, coagulation and quenching techniques are governed by solvent and heat diffusion, respectively, which are typically slow processes in high molecular weight systems, and can cause an undesirable skin/core morphology.
To summarize, various attempts by others to produce a homogeneous mixture of immiscible polymers or copolymers have involved processes such as solvent evaporation, coagulation, rapid freezing or freeze-drying. Each of these techniques is governed by a diffusive process, either of heat or of mass. Particularly in polymer-containing materials, diffusive processes are typically slow. The present invention produces a homogeneous mixture of immiscible polymers by density reduction of a solution whose solvent is above its critical point. The time scale of this process is governed by convection (i.e., bulk flow), not diffusion, and the characteristic convective velocity for flow of a compressible fluid such as a supercritical fluid is the speed of sound, which is quite rapid.
In addition to the foregoing, efforts have also been made to tailor polymer properties by bonding two or more dissimilar types of polymers in single, albeit high molecular weight, molecular species. Such polymers are of the general "block" or "graft" type material. Such polymers are quite different from random copolymers which can be prepared, for example, by the random copolymerization of two or more dissimilar polymerizable monomers. Rather, block or graft copolymers comprise large molecular units which individually comprise different polymerized species (referred to as "blocks") which are assembled into the final polymer structure by covalently bonding the blocks. Interestingly, however, the block or graft copolymers thus made can be in a nonhomogeneous molecular state at equilibrium. Stated differently, the blocks in the block or graft copolymer might be said to act almost independently of each other, as regards their phase and co-solubility behaviors. Considered thus, it seems reasonable that such polymers can exhibit inhomogeneity at equilibrium when their individual block units are, themselves, otherwise thermodynamically immiscible.
Producing homogeneous block or graft polymers is of practical interest for several reasons. The vast majority of pairs of polymers, or polymer blocks, are immiscible, and can form different phases at equilibrium. Pairs of polymers tend to macrophase separate or segregate; that is, they form separate domains of each component polymer, where the domains can be of the order of millimeters in size. Block (and graft) copolymers microphase separate; that is, they usually exhibit a microdomain morphology in the solid state, where the microdomain size is of the order of the block size. Because of phase separation, immiscible polymer blends are opaque and have poor mechanical properties, even to the point of having little mechanical integrity. In homogeneous miscible blends, however, transparency and good mechanical properties can be achieved. The properties are usually intermediate to the two components, so producing homogeneous mixtures of polymers offers the opportunity to achieve adjustable properties without the need for synthesizing new polymer materials. With nonhomogeneous block or graft copolymers the inhomogeneity results in properties of the original block constituents, plus unique properties which result from their connection.
Besides the usefulness of producing stable homogeneous polymer mixtures for the sake of their properties, it may be desirable to produce unstable homogeneous polymer mixtures that will revert in time to their thermodynamically stable, immiscible form. Since the rate of phase separation will be dependent on the environmental conditions, these materials could be used as environmental indicators, or as environmentally degradable materials. For block or graft copolymers, reversion to the thermodynamically stable and immiscible form results in the unique mechanical properties of the immiscible form.
According to the present invention, otherwise immiscible polymers such as block or graft copolymers are "homogenized" via a precipitation technique which is not limited by slow diffusion processes. The solid polymer is precipitated by rapid pressure (density) reduction from a homogeneous polymer solution in a supercritical fluid (SCF) solvent. Homogeneous polymers and polymer blends are produced thereby.